1. Field of the Invention
The present invention relates to a UWB (Ultra Wide Band) transmitter and receiver, and in particular it relates to a UWB transmitter and receiver which increase the transmission rate and improve the SN (signal-to-noise) ratio.
2. Description of the Related Art
A communication system using very short pulse signals for a so-called UWB system has been proposed (Refer to “Nikkei electronics,” Edition Mar. 11, 2002, Pages 55 through 66). The UWB system does not utilize any carrier waves or any intermediate frequency (IF), and it has features of a remarkably wide band and low consumption power, etc. As a major object of utilization of the UWB system, an ultra high-speed transmission among peripheral devices of a PC (personal computer) may be listed.
FIG. 3 is a view showing a configuration of a transmitter-receiver according to a related art super heterodyne system. A transmitting signal is converted to a high frequency signal by a mixer 21 with local oscillation signal from a voltage controlled oscillator (VCO) 22, is amplified in terms of high frequency by a power amplifier 23, is limited to a prescribed band by a bandpass filter 24, passes through an antenna switch 25, and is radiated from an antenna 26. When receiving, a high frequency signal received by the antenna 26 passes through the antenna switch 25, is limited to a prescribed band by the bandpass filter 27, is amplified by a low-noise amplifier 28, is made into an intermediate frequency signal by a mixer 29 by being mixed with local oscillation signal from the voltage controlled oscillator 22, is limited to a prescribed intermediate frequency band by the bandpass filter 30, is amplified by the amplifier 31, and is converted to a baseband signal which is a receiving signal by the mixer 32.
FIG. 4 is a view showing a configuration of a prior art UWB transmitter-receiver. A pulse generator 43 is controlled by a clock from a clock oscillator 41, and a transmitting signal is input into the pulse generator 43 into which a PN (Pseudo Noise) code from the PN code generator 42 is input. The pulse generator 43 generates, for example, a pulse train of {1, 1, −1, 1} with respect to a transmitting signal 1, and a pulse train of {−1, −1, 1, −1} with respect to a transmitting signal −1 (Direct Sequence Spread Spectrum). Herein, a bi-phase is employed, that is, a pulse having a positive polarity and a pulse having a negative polarity are used. The pulse train is limited to a remarkably wide prescribed band by a bandpass filter 44, passes through the antenna switch 45 without being converted in terms of frequency, and is radiated from the antenna 46. Radiated power is weak. However, sufficient information can be transmitted since the frequency band is remarkably wide and short-haul communications are assumed. When receiving, a pulse train signal which is received by the antenna 46 in an ultra wide band passes through the antenna switch 45, is limited to a prescribed band by the bandpass filter 47, and is amplified by a low-noise amplifier 48. A correlator 49 respectively detects a correlation between the received pulse train that is not converted in terms of frequency and the same pulse trains {1, 1, −1, 1} or {−1, −1, 1, −1} as those in the pulse trains used for transmission from the pulse generator 43, and the correlation output is amplified by an amplifier 50, wherein if the correlation between the received pulse train and reference pulse trains is sufficiently large, a receiving signal 1 or −1 can be obtained.
The transmission speed of the UWB system depends on a multipath (=echo) in a communication path. If an echo exists, it is difficult to distinguish which pulse is a true transmitting signal. Therefore, usually, the chip period (pulse repetition period) is defined so as to generate a next pulse at the intervals to make the size of the echo sufficiently small. Thus, the transmission rate is limited by the echo.